Many electrophotographic printers, such as laser printers, take advantage of basic principles of static electricity to provide a fast and accurate way to print data onto a suitable print medium. An example of a monochrome laser printer 10 is illustrated in FIG. 1. The monochrome printer 10 uses a laser unit 12 to shine a laser beam against a photosensitive drum assembly 14. Before the photosensitive drum assembly 14 is exposed to the laser beam, a uniform positive charge is applied to the drum assembly 14 by a corona wire 16. As the drum assembly 14 rotates, a rotating mirror 18 directs the laser beam across the length of the drum assembly 14 and the portions of the drum assembly 14 illuminated by the laser beam are thereby discharged, leaving a negatively charged electrostatic image. The drum assembly 14 continues along its rotational path and passes a toner assembly 20 having a toner container 22 and a developer roller 24 coated in positively charged toner particles. The negative charge of the electrostatic image on the drum assembly 14 overcomes the weaker negative charge of the developer roller 24 so that toner adheres to the electrostatic image on the drum assembly 14. A piece of paper or other printable medium is then negatively charged and rolled against the drum assembly 14 so that it pulls away the toner, and thus the image, from the drum assembly 14. To affix the image, the paper runs through a fuser 26 that heats the paper. The toner, which is typically a mix of a plastic and pigment, becomes affixed to the paper as the plastic particles melt.
Color laser printers often use the same static electricity principle as monochrome printers, but color printers generally use various proportions of several different colors of toner to achieve the desired print color. A standard set of toner colors employed by a color laser printer to produce a broad palette of colors is: cyan (C), yellow (Y), magenta (M) and black (K). Each of these toner colors needs to be individually applied. Two typical configurations for applying the toner colors in color laser printers are the multipass format and the in-line format.
Multipass color laser printers generally operate by repeatedly passing a piece of paper, or other suitable print medium, by a single photosensitive drum and switching the color toner assembly for each pass. For example, to convert the monochrome printer of FIG. 1 into a multipass color printer the toner assembly 20 of FIG. 1 may be replaced with a carousel of four (C, Y, M and K) toner assemblies and the paper would be passed by the drum assembly 14 four separate times, each time with a different toner assembly rotated into place for the pass. This type of multipass arrangement leads to a monochrome print speed that is four times as fast as the color print speed due to the need for four separate passes of the paper in a color print job and only one pass for the monochrome (e.g. black) print job.
In contrast to the multipass format, an in-line color laser printer operates to move the paper in a single path that travels by each of the four sequentially positioned toner assemblies. This arrangement tends to greatly increase the speed of color printing because repeated cycles are avoided. This arrangement also reduces the perceived speed of the monochrome (K) printing because, due to the same path taken by the paper whether the printer performs a color or monochrome (e.g. black only) print job, there is not necessarily a difference between the color and monochrome print speeds.
Techniques for increasing printer speed include spinning the mirror used to reflect the laser beams onto the electrically charged drum at a faster rate, or using mirrors with an increased number of sides. There tend to be mechanical limits on how fast a mirror can be spun. Also, multi-sided mirrors can be more expensive to manufacture and use. Accordingly, it would be advantageous to increase monochrome printing speed in color laser printers, such as in-line printers, in a more cost efficient manner.